The present invention relates to reserving resources in a cellular radio communications system. One example and non-limiting application of the invention relates to advance reservation of data processing and memory resources needed to accommodate probable handover operations for a mobile radio connection.
In a cellular radio communications system, a handover operation allows an established radio connection to continue when a mobile radio participating in that connection moves between cells in the system. Handover is typically initiated when the signal strength or signal quality of the radio connection with an origination base station falls below a predetermined threshold value. Often, a low signal strength or a poor signal quality indication means that the mobile station is near a border between two cells. If the mobile station moves closer to a destination cell or to a clearer line of unobstructed sight, handover of the radio connection to the destination cell usually results in improved radio transmission and reception.
In some cellular systems, a handover operation requires physically breaking the connection with the origination cell and then re-establishing the connection with the destination cell, i.e., a xe2x80x9cbreak-before-makexe2x80x9d switching operation. Such xe2x80x9chardxe2x80x9d handover techniques are typically employed in Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) type cellular systems. On the other hand, xe2x80x9csoftxe2x80x9d handover techniques may be employed in Code Division Multiple Access (CDMA) type cellular systems. CDMA is an increasingly popular type of access for cellular communications because a higher spectrum efficiency is achieved compared to FDMA and TDMA techniques which means that more cellular users and/or services can be supported. In addition, a common frequency band allows simultaneous communication between a mobile station and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the mobile stations. Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately received at the receiving station. The high speed PN modulation also advantageously allows the receiving station to generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal.
In CDMA, therefore, a mobile station need not switch frequency when handover of a connection is made from one cell to another. As a result, a destination cell can support a connection to a mobile station at the same time the origination cell continues to service the connection. Since the mobile station is always communicating through at least one cell during handover, there is no disruption to the call. Hence the termxe2x80x94xe2x80x9csoft handover.xe2x80x9d In contrast to hard handover, soft handover is a xe2x80x9cmake-before-breakxe2x80x9d switching operation.
FIG. 1 is a high level diagram of a radio communications system 10 showing a soft handover operation. A radio network controller (RNC) 12 is coupled to adjacent base stations 14 and 18. Base station 14 serves a cell area 16, and base station 18 serves a cell area 20. Mobile stations 22 and 24 are located within cell 16, and mobile station 26 is located in cell area 20. Because mobile station 24 is near the border between cells 16 and 20, it has established communication links P1 and P2 with both base stations 14 and 18 which simultaneously support the connection with the mobile station 24. When a mobile station is in soft handover between two base stations, a single signal is created at the mobile station receiver from the two signals transmitted by each base station using a RAKE demodulation combination process. Those two signals are generated by the RNC xe2x80x9csplittingxe2x80x9d or broadcasting a downlink signal intended for the mobile station into two parallel identical signals with one being directed to the origination base station 14 and the other to the destination base station 18. In the opposite xe2x80x9cuplinkxe2x80x9d direction, the mobile station transmitter broadcasts the signal to both base stations, and the signals are combined in the RNC 12. More than two base stations may be involved in a soft handover.
A similar operation may occur between sector cells of a common base station that employs plural antennas. The radio communications system 10 in FIG. 2 shows a base station 30 coupled to RNC 12 having multiple sectors Sec 0-Sec 5 where each sector includes one or more sector antennas. Mobile station 32 is located on the border of sectors 0 and 1. Demodulation elements at the base station 30 demodulate mobile station signals received at both sectors 0 and 1. Combining the demodulated mobile station signals from sectors 0 and 1 at the base station permits xe2x80x9csofter handoverxe2x80x9d to take place. In other words, the mobile connection is supported by a destination sector before an origination sector no longer supports the connection.
Accordingly, soft and softer handover are highly desirable features of a mobile radio communications system based on spread spectrum CDMA because they offer make-before-break switching of a connection and also because they offer diversity combining of plural paths of the same signal. Diversity combining combats fading and interference. However, system resources must be allocated in order to carry out handover operations. In soft handover, for example, diversity handover units (DHOs) located in the RNC perform macro diversity combining of the connection information in the uplink (mobile-to-base) direction and macro diversity splitting of the connection information in the downlink (base-to-mobile) direction. Moreover, a single DHO entity (an entity may be implemented using software and/or hardware) may be employed for each service provided to a mobile station, i.e., a call may include several services like voice, video, and data services in a multimedia call. Because the number of DHO entities required to support a connection varies depending on the call, it is considered a dynamic service parameter. Services may also specify at the time of request certain radio interface type parameters like a particular bandwidth, e.g., peak or average bit rate, or a particular delay, e.g., maximum tolerable delay. These types of parameters are considered static. Ultimately, software and hardware resources must be allocated to support both dynamic and static service parameters. At a basic resource level, data processing and memory resources are required to support service parameters associated with a call connection with the mobile station.
Higher level resources like CDMA spreading codes and lower level resources like data processing and memory can be allocated at the time of a call setup for a requested service or at the time a known service is added or removed from a call by matching those resources needed for the requested service(s). On the other hand, there are other unknown services or services that are not explicitly requested that nevertheless require hardware and software resources. For example, a number of handover paths ultimately used to support a mobile station connection is not specified or known at the time of call setup. Indeed, the number of handover paths will likely vary depending upon the mobile station""s location and on the current radio conditions in the mobile communications network. A mobile station that is in the center of a particular cell will likely employ fewer handover paths, and therefore, fewer associated resources are needed to support those paths as compared to a mobile station traveling to or located near the border between two or more cells. A mobile station in this latter situation will likely require more resources to support plural handover paths for a mobile connection.
To account for unspecified or unknown resources that nevertheless may be needed to support the connection at sometime during its life, a worst case resource reservation/allocation could be made for each connection at setup. If resources were unlimited, a worst case resource reservation/allocation would be a satisfactory solution despite being inefficient. But in the real world, resources are costly and/or limited, and efficiency is important. Accordingly, it is an objective of the invention to efficiently allocate a proper amount of resources (e.g., enough but not too many) to support the needs of a particular mobile connection.
Rather than suffering the inefficiency of overallocating resources in a worst case manner for each call, resources could be allocated in real time when needed. The problem with this approach is the delays that are inherently a part of such a real time resource allocation approach. In overload situations, if the resources are not available when needed and will not be in the foreseeable near future, it may be necessary to drop the call. It is therefore also an objective of the present invention to efficiently allocate resources in a timely fashion that keeps delays to a minimum.
The present invention overcomes these resource allocation problems and meets the above-stated and other objectives by predicting the amount of resources that will likely be necessary to support a connection with a mobile station before those resources are actually required. An unknown value of a dynamic connection parameter, like the number of radio paths likely to be involved in supporting the connection, is predicted. In the handover context, these radio paths might correspond to paths with different base stations (as in hard and soft handover) or to paths with different base station sectors (as in softer handover). The underlying resources are allocated using the predicted connection parameter and include for example data processing and memory hardware and software resources, radio resources, etc.
In a preferred example embodiment, the predicted connection parameter includes a number of diversity paths likely to be involved in supporting a connection in a CDMA cellular communications system, and the resources include CDMA spreading codes, diversity handover units (DHOs), data processing units, memory units, etc. For ease of description, an amount of resources may sometimes simply be defined generally in terms of xe2x80x9cunits.xe2x80x9d Of course, other predicted connection parameters and other resources may be included as well. An average number of diversity paths (and preferably a moving average) is determined based upon a number of diversity paths currently supporting other active mobile connections.
In another preferred example embodiment, resources are allocated based both on one or more xe2x80x9cdynamicxe2x80x9d connection parameters unknown at the time the connection is set up and on one or more xe2x80x9cstaticxe2x80x9d connection parameters known when the connection is set up. For example, a xe2x80x9cdynamicxe2x80x9d connection parameter includes a number of supporting paths likely to be used to support the connection. A static connection parameter includes (in this example) to a bandwidth or a maximum delay requested by a service associated with the connection.
The present invention may be implemented in a control node in a radio communications network where mobile stations communicate with the radio network via base stations over a radio interface. Each base station is associated with at least one geographic cell area. The control node includes a communications controller that initiates establishment of a connection between the radio communications network and a mobile station. The control node further includes a resource controller coordinating with the communications controller to allocate resources to support the connection based upon a predicted connection parameter, e.g., a predicted number of diversity handover paths, that may be involved in supporting a connection.
In the diversity handover path connection parameter example, the resource controller determines the predicted number of paths based upon a number of current paths per mobile station with plural base station cells for active connections being supported in the radio network. The plural base station cells may be associated with one base station (a cell is associated with a base station sector) or with plural base stations (each cell is associated with a base station). If the resource controller is located in a base station, the paths correspond to different base station sectors. Alternatively, the resource controller may be located in a radio network controller coupled to plural base stations where the paths correspond to different base stations.